Wireframe is a method of rendering a 3D object. The wireframe method shows the object as if it was made of thin wires without solid faces to cover what lies behind them. Wireframe rendering is very fast compared to solid-faced rendering but can be confusing due to the increased number of lines visible. Also, there is no occlusion culling done (but some culling and LOD are done), which in turn adds even more lines to the scene. In SL 18.104.22.168, the wireframe option has been reintroduced to Debug > Rendering > Wireframe (Ctrl-Shift-O) after being disabled since version 1.6(?).
[Images of a scene in solid render and wireframe pending.]